The advent of anti-lock brake systems (ABS) and the placement of speed sensing devices at each of the wheels has sparked efforts to develop reliable methods for detecting tire deflation based on wheel speeds measured during driving. Theoretically, four equally inflated tires on a vehicle will have substantially the same rolling radius and will therefore each rotate at substantially the same speed during straight-line driving on a dry, flat and uniform surface. When a tire becomes deflated, its rolling radius is reduced and the wheel speed increases to compensate for the smaller radius. Numerous methods of detecting deflated tires based on the rolling radius concept have been created. These methods monitor the wheel speeds and detect variations that may be attributable to tire deflation.
The rolling radius effect is not the only influence acting on a tire. For any tire, there is also a grip-rate effect. Simply stated, the grip-rate of a tire is defined as the ratio of the longitudinal tractive force (i.e., torque) applied to the wheel versus the longitudinal tire slip. For a normally inflated tire, the grip-rate remains substantially constant and there is a direct correlation between the tractive force applied to the wheel and the corresponding slip experienced by the tire. As a tire becomes deflated, however, the pressure at the contact patch is lowered and the grip-rate is increased. The lower pressure causes the tire to grip the road better and slip less than a normally inflated tire.
When analyzed with respect to the driven wheels of the vehicle, the grip-rate effect causes problems for tire deflation detection methods that use the rolling radius concept. In situations when a high accelerative tractive force is exerted on a deflated tire (driving with full throttle up a hill or operating the vehicle at high or maximum speeds), the increased grip-rate causes the wheel to spin slower than it would if its tire were normally inflated and slipping more. Recall, however, that a wheel with a deflated tire also has a smaller rolling radius, causing it to spin faster than a wheel with a normally inflated tire. Therefore, under high accelerative tractive forces, the tendency toward slower wheel speed due to the increased grip-rate often reduces or cancels out the tendency toward increased wheel speed caused by the smaller rolling radius of a deflated tire, causing the net observed wheel speed to be identical to that of a normally inflated tire. When this occurs, a deflated tire may go undetected.
Another influencing factor that creates problems for methods based on the rolling radius concept is the variety of loading conditions that the tires can experience. Depending on the position of the load in a vehicle, it is often common to have one or more tires that are loaded more heavily than the other tires. Normally inflated tires that carry a heavier load will have a smaller rolling radius than normally inflated tires carrying a lighter load. Therefore, the wheels carrying the heavier load will have a higher rotational speed than the wheels carrying the lighter load. Since most rolling radius detection methods compare the wheel speeds to one another, the effects of asymmetrical loading are often misinterpreted as a deflated tire.
In prior art methods that use the rolling radius concept, the problems associated with the grip-rate effect are normally addressed by tuning the system to be more sensitive, while the problems associated with the asymmetrical loading effect are normally addressed by tuning the system to be less sensitive. A compromise sensitivity level that adequately compensates for both problems is frequently impossible to achieve.
Several attempts have been made to address the effects of grip-rate changes and/or the effects of asymmetrical loading. U.S. Pat. No. 5,936,519 discloses a rolling radius method for detecting tire pressure drop wherein the deflation detection calculation (known as a "sum of the diagonals" calculation) only utilizes data collected when the tractive effort on the tire is low, that is, when the vehicle is decelerating without application of the brake. In other words, the method copes with the grip-rate effect by filtering out the data that is potentially flawed by the grip-rate influence. This discriminating method simply eliminates the problematic data commonly collected during high speed driving or uphill driving. By eliminating this data however, the deflation detection method is not particularly robust as it simply ignores potentially valuable data.
Other prior art tire deflation detection methods have attempted to incorporate the grip-rate concept as part of the deflation detection process. The main engine of these methods is the inherent correlation between the driven wheel force and the driven wheel slip as described above.
U.S. Pat. No. 5,561,415 discloses a method for determining pressure loss in tires on a driven axle by correlating simultaneously collected wheel slip values and wheel drive force values. The method seeks to detect when the overall or aggregate grip-rate of the driven axle has changed. If the drive force values for the driven axle are not available, acceleration data is used to estimate the drive force. The correlated data is compared to a predetermined characteristic curve to determine whether a tire on the driven axle is deflated.
This method is problematic in that the two driven wheels are not treated independently. Rather, the two driven wheels are treated in the aggregate which can result in faulty deflation detection. Specifically, a relatively negligible inflation loss in each tire on the driven axle can be interpreted as a single critically deflated tire. Additionally, when the drive force is estimated, assumptions regarding payload mass must be made. These assumptions do not consider the effects of asymmetrical loading. False detection or failure to detect may result. While the method does discuss the possibility of using measured payload values, payload adjusting values, or manually input approximate payload values, such corrections involve additional burdensome steps.
U.S. Pat. No. 5,747,686 also discloses a method of detecting a deflated tire based on the correlation between drive force and drive slip. The driven wheel torque is calculated and used to predict what the driven wheel slip should be in light of the direct correlation. The actual driven wheel slip is then calculated from wheel velocity data and compared to the predicted driven wheel slip to determine if there is a deflated tire.
This method is problematic in that the left side tires are treated independently of, and isolated from, the right side tires. The tires of each side of the vehicle are treated in the aggregate which can result in faulty deflation detection. For example, a relatively negligible overinflation of the left follower tire and a relatively negligible deflation of the left driven tire can be interpreted as a critically deflated left side driven tire.
In addition to the aggregation problems discussed above, the prior art grip-rate methods are also susceptible to false detection or failure to detect due to tire changes. Different tire models have different nominal grip-rates such that installation of a different tire, having a new nominal grip-rate, may fool the prior art methods, which, to one extent or another, measure grip-rates relative to a fixed value stored in the program.